With the advances in the human genome project, many disease causing genes have been identified. As a result, the administration of [genes] as drugs into the human body for effective treatment, so called, [gene therapy], has become attractive (non patent reference 1). In gene therapy, the tools which introduce therapeutic genes into cells requiring treatment are called [vectors], and virus and plasmid vectors are mostly being used for real clinical studies at the present time.
Viruses deficient in pathogenicity are typically used as viral vectors. In this case, the function of the virus by which it transfers its own genes into cells by infection is utilized. Therefore, these vectors have an advantage in that they can transfer genes more efficiently into cells compared with other vectors. However, viral vectors on rare occasions can be contaminated with viruses which have escaped from pretreatment (inactivation) steps performed to remove their pathogenicity, and become pathogenic and proliferate freely as side effects. Furthermore, they have considerable problems including the occurrence of unexpected genetic recombination and the possession of immunogenicity.
Plasmid DNA (pDNA) is becoming attractive as an alternative vector in recent years, with low immunogenicity compared to viruses, and high productivity. However, recent research has revealed that if the sequence known as the CpG motif exists in the plasmid DNA, macrophage or dendritic cells can recognize the motif as a stress signal, causing induction of an immune activation reaction including the production of various inflammatory cytokines. Inflammatory cytokines lower gene expression because of their cytotoxicity; therefore, they are not suitable for gene therapy. Furthermore, plasmid DNAs usually contain antibiotic resistance genes and extra gene sequences derived from other species such as bacteria. These genes are not only undesirable for gene therapy but also have a potential to cause side effects, such as expression of undesirable proteins coded by these genes, and production of abnormal genomic DNA by incorporation of these genes into normal genomic DNA (non-patent reference 2).
Gene transfer technology (MIDGE technology) using a Minimalistic Immunogenically Defined Gene Expression (MIDGE) vector, that is, a dumbbell-shaped DNA is disclosed in the description of U.S. Pat. No. 6,451,593. This vector is the template DNA for RNA transcription, and the DNA is a circular stranded DNA which is able to form a dumbbell shape.
The circular strand of this dumbbell-shaped DNA vector comprises a first complementary sequence, a first noncomplementary sequence, a second complementary sequence, and a second noncomplementary sequence. The first and second complementary sequences pair to form a double strand. The double strand contains a promoter sequence, a coding sequence, and a polyA or a stabilizing sequence. Moreover, the first and second noncomplementary sequences form single-strand-loops. (patent reference 1).
A dumbbell-shaped DNA vector was constructed as a superior alternative vector which reduces the disadvantages and enhances the advantages of viral and plasmid vectors (non-patent reference 3). The dumbbell-shaped DNA contains only promoter and transcription sequence regions, as shown in FIG. 1. Therefore, immunogenicity can be minimized because it does not contain extra sequences. Furthermore, the closed circular DNA which is produced by creating loops at both ends of the desired gene sequence is not affected by Exonuclease (Exo-type nucleic acid digestive enzyme) activity when it is transfected into cells. Thus, it is known that the dumbbell-shaped DNA is resistant to digestion and relatively stable in serum and cells. Moreover, a method is known for producing a dumbbell-shaped DNA containing only necessary and minimal gene sequences for gene therapy by ligation of three molecules, as shown in FIG. 6 (non-patent reference 4). In the general method for producing the dumbbell-shaped DNA shown in FIG. 6, the linear-shaped target DNA fragment is amplified by PCR (step 1 (PCR) in FIG. 6) using cDNA containing a target gene sequence as a template DNA, and then both ends of the linear-shaped target DNA fragment are digested with restriction enzymes. In the general method, this fragment and two synthesized DNA fragments which contain the loop region of the dumbbell are ligated by DNA ligase to construct the dumbbell-shaped DNA.
However, a disadvantage of this production method is that the ligation reaction (step 4 in FIG. 6 (intermolecular ligation)) involves ligation of three molecules, and moreover, ligation efficiency is low because the reaction is an intermolecular ligation reaction. Furthermore, the molar concentration of each of the two synthesized DNA fragments containing the loop regions of the dumbbell needs to be in a large excess compared to the molar concentration of the linear-shaped target DNA fragment. Therefore, almost all the synthesized DNA containing the loop regions is wasted without being ligated. Moreover, this can hamper the purification process. As a result, recovery of the target dumbbell-shaped DNA becomes low.
Another method for producing the dumbbell-shaped DNA vector in vitro is as follows;
As shown in FIG. 7, two inverted N.Bpu10I nickase recognition sequences are connected at both ends of the linear-shaped target DNA fragment, using cDNA as a template. The fragment is then subcloned into a plasmid vector DNA which is amplifiable in E. coli and the resulting vector is then transformed into E. coli for large scale amplification. Then, treatment as in 6˜9 in FIG. 7 is performed to construct the dumbbell-shaped DNA vector containing a target DNA, and the dumbbell-shaped DNA derived from the plasmid vector is digested and purified. However, the disadvantage of this production method is that the process is very complicated and time consuming because it requires processes such as subcloning and digestion of the plasmid vector derived dumbbell-shaped DNA.    Patent Reference 1: U.S. Pat. No. 6,451,593 Specification    Non-patent Reference 1: Verma, I. M.; Somia, N. Nature 1997, 389, 239-242.    Non-patent Reference 2: Luo, D.; Saltzman, W. M. Nat. Biotechnol. 2000, 18, 33-37. Ferber, D. Science 2001, 294, 1638-1642. Medzhitov, R. Nat. Immunol. 2001, 2, 15-16.    Non-patent Reference 3: Schakowski, F.; Gorschluter, M.; Junghans, C; Schroff, M.; Buttgereit, P; Ziske, C; Schottker, B.; Konig-Merediz, S. A; Sauerbruch, T; Wittig, B.; Schmidt-Wolf, I. G. Mol. Ther. 2001, 3, 793-800.    Non-patent Reference 4: Zanta, M. A.; Belguise-Valladier, P.; Behr, J. P. Proc. Natl. Acad. Sci. USA 1999, 96, 916.